Silicates such as alkali metal silicates are extremely effective in protecting metals and ceramics against the corrosive effects of alkaline solutions. Moreover, such silicates also provide useful detergent boosting effects when present in cleaning solutions. Accordingly, alkali metal silicates are widely used in a broad range of detergent products such as automatic dishwashing detergents, metal cleaners and laundry detergents.
Another important usage of alkali metal silicates is as an additive in aqueous cleaning solutions used to remove solder flux and other residues from electronic circuit assemblies such as printed circuit boards or printed wiring boards. As described in commonly-assigned co-pending application, U.S. Ser. No. 731,512, filed Jul. 17, 1991, it has been found that solder fluxes contained on electronic circuit assemblies, such as printed circuit boards and printed wiring boards, can be effectively removed by contact with aqueous solutions of alkaline salts such as alkali metal carbonates and mixtures of alkali metal carbonates and bicarbonates. It has further been found that the addition of alkali metal silicates to such formulations greatly improves the corrosion protection of ceramic and metal components on such electronic assemblies.
Accordingly, while the improved aqueous cleaning compositions of this invention are useful in many and varied types of cleaning compositions, such as dishwashing detergents, laundry detergents and the like, the compositions of this invention are particularly useful in removing solder flux and other residues from electronic circuit assemblies such as disclosed in the above-mentioned commonly assigned application.
The cleanliness of electronic circuit assemblies (ECA), such as printed circuit boards (PCB) or printed wiring boards (PWB), is generally regarded as being critical to their functional reliability. Ionic and nonionic contamination on circuit assemblies is believed to contribute to premature failures of the circuit assemblies by allowing short circuits to develop.
In the manufacture of electronic circuit assemblies, ionic and nonionic contamination can accumulate after one or more steps of the process. Circuit assembly materials are plated, etched, handled by operators in assembly, coated with corrosive or potentially corrosive fluxes and finally soldered.
In the fabrication of electronic circuit assemblies, e.g., printed circuit boards, soldering fluxes are first applied to the substrate board material to ensure firm, uniform bonding of the solder. These soldering fluxes fall into two broad categories: rosin and non-rosin, or water soluble, fluxes. The rosin fluxes, which are generally only moderately corrosive and have a much longer history of use, are still widely used throughout the electronics industry. The water soluble fluxes, which are a more recent development, are being used increasingly in consumer products applications. Because water soluble fluxes contain strong acids and/or amine hydrohalides, such fluxes are very corrosive. Unfortunately, residues of any flux can cause circuit failure if residual traces of the material are not carefully removed following soldering and thus remain on an electronic circuit assembly.
While water soluble fluxes can be easily removed with warm, soapy water, the removal of rosin flux from printed circuit boards is more difficult and has therefore traditionally been carried out with the use of chlorinated hydrocarbon solvents such as 1,1,1,-trichlorethane, trichloroethylene, trichloromonofluoromethane, methylene chloride, trichlorotrifluoroethane (CFC113), tetrachlorodifluoroethane (CFC112) or mixtures or azeotropes of these and/or other solvents. These solvents are undesirable, however, because they are toxic and when released into the environment deplete the ozone layer and/or contribute to the greenhouse global warming effect. Thus, use of such solvents is subject to close scrutiny by the Occupational Safety and Health Administration (OSHA) and the Environmental Protection Agency (EPA), and stringent containment equipment must be used. Moreover, if released into the environment these solvents are not readily biodegradable and are thus hazardous for long periods of time.
Alkaline cleaning compounds known as the alkanolamines, usually in the form of monoethanolamine, have been used for rosin flux removal as an alternative to the toxic chlorinated hydrocarbon solvents. These high pH compounds (e.g., about 12 pH), chemically react with rosin flux to form a rosin soap through the process of saponification. Other organic substances such as surfactants or alcohol derivatives may be added to these alkaline cleaning compounds to facilitate the removal of such rosin soap. Unfortunately, these compounds, as well as the water soluble soldering fluxes, have a tendency to cause corrosion on the surfaces and interfaces of printed wiring boards if such compounds and fluxes are not completely and rapidly removed during the fabrication process.
In other approaches, Daley et al., U.S. Pat. No. 4,635,666 utilize a highly caustic solution having a pH of 13 in a batch cleaning process. This method severely oxidizes the solder applied to the circuit board. In Hayes et al., U.S. Pat. Nos. 4,640,719 and 4,740,247 rosin soldering flux and other residues are removed from electronic assemblies by means of terpene compounds in combination with terpene emulsifying surfactants by rinsing in water.
The complete removal of adhesive and other residues also poses a problem. During the manufacture of electronic circuit assemblies the components are mounted on the upper surface of the board with leads protruding downwardly through holes in the board and are secured to the bottom surface of the board by means of an adhesive. Further, it is sometimes necessary to temporarily protect certain portions of the board from processing steps such as the process of creating corrosion resistant gold connecting tabs at the board edges. This transient protection of portions of the circuit board can be achieved by the application of special adhesive tape to susceptible areas. Once such protection is no longer needed, the adhesive tape must be removed. In both instances, a residue of adhesive generally remains which, if not thoroughly removed, can cause premature board failure. Removal of this adhesive residue has traditionally been carried out by the use of chlorinated solvents which, as already described, are toxic and environmentally undesirable.
Thus, the residual contaminants which are likely to be found on electronic circuit assemblies and which can be removed by the compositions and method of the present invention include, but are not limited to, for example, rosin flux, photoresist, solder masks, adhesives, machine oils, greases, silicones, lanolin, mold release, polyglycols and plasticizers.
The cleaning compositions for electronic circuit assemblies as described in the above-mentioned commonly assigned application have a relatively low pH compared to prior art aqueous circuit board cleaners which utilize high pH, i.e., 12 and above, aqueous cleaning compositions. These high pH solutions are very corrosive on the circuit assemblies and are relatively unsafe to use. The prior art cleaners also yield high biological oxygen demands and chemical oxygen demands in the water effluents from the cleaning process. Unfortunately, as the pH is lowered, any alkali metal silicate which is present in the cleaning composition either to improve detersive action or, more particularly, as an anti-corrosion agent tends to flock out of the solution. It would be very advantageous to include alkali metal silicate in the alkaline aqueous cleaning compositions for electronic circuit assemblies in order to protect the ceramic and metal components of the circuit assemblies from the corrosive action of the alkaline metal cleaning salts. At the same time it would be advantageous to maintain a relatively low pH in the cleaning solutions so as to minimize the corrosive effects thereof and to ensure safety to humans resulting from accidental exposure to the product or solutions thereof. Many household cleaners have relatively low pH's and would be enhanced by the addition of stabilized silicate corrosion inhibitors.